The present invention relates to offshore drilling applications and, more particularly, to systems and methods of effectively wiping and displacing a deep water riser prior to disconnection from a blowout preventer.
In offshore drilling applications, risers are used as a temporary fluid conduit that communicably couples a subsea wellhead installation, including a blowout preventer, to a drilling facility on the surface, such as a platform or other type of submersible or semi-submersible drilling rig. In operation, risers generally provide a means of circulating drilling fluid, and any additional solids and/or fluids, between the wellbore being drilled and the surface.
During the course of drilling an offshore well, it may be required to disconnect the riser from the wellhead on multiple occasions. For example, during tropical depressions or hurricanes, or other extreme weather conditions, the waves in the ocean can heave up to and exceed fifty feet in depth/height. In such conditions, it is often advisable to disconnect the riser from the wellhead in order to avoid damage to the wellhead and/or the riser string. Disconnecting the riser from the wellhead requires the proper displacement (i.e., removal) and containment of the drilling fluid present within the riser which, if inadvertently discharged directly into the surrounding oceanic environment, could present serious environmental concerns, not to mention fines potentially levied on the operator.
One way of safely removing the drilling fluid from the riser for proper containment is to drop what is known as a wiper plug into the riser until it reaches the wellhead. Upon reaching the top of the wellhead, the wiper plug is then activated, which, in some cases, forces multiple annular sealing elements against the inner wall of the riser and thereby serves as a separation point between the fluids above and below the wiper plug within the riser. The wiper plug is then pumped back to the surface using a spacer fluid injected into the riser at a location below the wiper plug, thereby forcing the wiper plug to ascend the riser string and simultaneously displacing the drilling fluid out of the riser. In most applications, the spacer fluid is seawater, and pumping the wiper plug to the surface fills the riser below the wiper plug with seawater. Upon disconnecting the riser, the seawater spacer fluid can be discharged directly into the ocean with little or no environmental impact.
At least one problem with conventional wiper plugs, however, is that they are typically pumped out of the riser and subsequently deposited into a moon pool or wet porch of the drilling facility at the surface. The wiper plugs must then be retrieved from the moon pool, which is often a very dangerous and difficult task, as can be appreciated by those skilled in the art. Moreover, conventional wiper plugs are not able to be quickly removed from the riser in the event of an ensuing emergency which may require immediate detachment of the riser from the wellhead installation. Instead, conventional wiper plugs are, for the most part, dependent on fluid pressure from the surface, which could take a great deal of time to advance the wiper plug through the entire length of the riser string.